Typical drilling operations employ a number of techniques to gather information such as the depth and inclination of a borehole and the types of rocks through which a drill pipe and drill bit are drilling. For this purpose, techniques called Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD) were developed in the oil exploration and production industry. These techniques enable the collection of data in real-time. LWD collects logging information similar to the conventional wireline logging, while MWD also enables a driller to determine the position and orientation of the drill bit and direction of a borehole during the drilling operation so that the driller can more accurately control the drilling operations. For the purpose of the following description, these and similar techniques will be collectively referred to as “MWD”.
Common to MWD techniques is the problem of transmitting data from the bottom of a borehole to a point on the surface where it can be collected and processed. A typical technique for this type of data transmission is mud pulse telemetry. During the drilling operation, drilling mud is pumped from a mud pump downward through the drill pipe and emerges near the drill bit at the bottom of the drill hole. This mud cools and lubricates the drill bit, carries rock cuttings to the surface where they can be analyzed and prevents the walls of the borehole from collapsing.
In mud pulse telemetry, a transmission device, or “pulser,” such as an electromechanical pulser or a mud siren near the drill bit generates an acoustic signal that is transmitted upward to the surface through the downward traveling column of mud. Modern mud sirens, for example, are capable of generating a carrier pressure wave of 12 Hz. A transducer, typically at the surface, receives the signal and transmits it to a signal processor. The signal processor then decodes and analyzes the signal to provide information about the drilling operation to the driller.
A major problem with decoding and analyzing the signal is that noise seen by the transducer, generated by the drilling operation, obscures the signal. There are a number of potential sources of noise generated during MWD. Noise may be introduced by the turning of the drill bit and drill pipe and/or from the mud pump used to force the mud into the drill pipe. Another source of noise is a reflected signal that is created when the original signal hits a pulsation dampener, or “desurger”, near the top of the mud column and is reflected back down the hole. In addition to noise, the MWD/LWD signal may be degraded by the type of mud, the mud pressure, the length and changes in diameter of the drill pipe and its joints, and the pulsation dampener. Of these potential and actual sources of noise, the noise generated by the mud pumps is often considered to be the one that interferes most dominantly with the signal.
The mud pump has two mechanisms of generating pressure fluctuation. The first is through the so-called “water hammer” effect due to imperfect synchronization of inlet/outlet valves at the beginning and end of each piston traversing cycle. This tends to produce pressure pulses of large amplitude and short duration. The second mechanism is through the pulsating nature of the flow generated by such pumps generating harmonic noise at each piston stroke.
To obtain reliable MWD signal decoding, slow data transmission rates are typically used (about 1 to 10 bit(s) per second) in order to sustain an acceptable signal-to-noise (S/N) ratio. If data transmission rates are increased, clock tracking and timing recovery, and the S/N ratio between the pulser and transducer become very sensitive and difficult to maintain due to the nature of the drilling operations, thus, decreasing the reliability of the MWD data.
Numerous techniques have been developed to reduce the effects of the noise sources on the signal, which can be broadly categorized as signal processing, including the use of differential measurements, signal amplification and/or repetition and mechanical noise attenuation methods.
Present systems rely mainly on existing pulsation dampeners to reduce pump noise and on signal processing software to counter the noise effect. The performance of the gas charged dampener depends on the pre-charge pressure value and it varies as the mud pipe pressure changes. Moreover, the known desurgers or dampeners, as found in the oilfield industry, tend to have very short throat sizes of the diameter of the flow pipes they are connected to. As furthermore the gas charge of the desurger is set without knowledge of the telemetry signal, it does not contribute efficiently towards reducing the noise in the telemetry signal band. It is often found that performance of the desurger deteriorates as the pressure increases, particularly beyond 2000 psi [13.78 MPa]. The performance of the reactive dampeners (no gas) is independent of operating pressure. However, very large physical size is required to achieve sufficient dampening. For instance, increasing the size of a reactive dampener (PPC Inc.) from 180 to 240 gallon [681 to 908 liters] results in peak-peak noise reduction from 105 to 80 psi [0.72 to 0.55 MPa].
Noise cancellation by signal processing means has been successful in many applications. However in some cases, the selection of correct parameters by experienced personnel is required to obtain optimal result. Whichever signal processing method is chosen, attenuation of noise by physical filters will make the task of further signal processing easier.
It is therefore an object of the present invention to reduce the noise level in mud pulse telemetry, particularly the noise generated by the mud pump, through mechanical filtering.